1. Field of the Invention
The present invention relates to a high-sensitivity and high-definition positive photoresist composition which is especially suitable for lithography using i-ray, and to a multilayer resist material using the composition.
2. Description of the Related Art
Positive photoresist compositions each containing an alkali-soluble resin and a quinonediazide ester have satisfactory definition, sensitivity, and etching resistance, and therefore have been applied in practice to the production of semiconductor devices and liquid crystal display devices.
With increasing demands for minitualized and high-capacity semiconductor integrated circuits (ICs), there is a strong felt need to form resist patterns in a design rule of 0.35 .mu.m or below, more particularly 0.30 .mu.m or below.
Even in such an ultrafine design rule, there are demands for photoresist compositions which can form resist patterns satisfactory in sensitivity, exposure margin, and focal depth range properties and having a good shape.
Such a resist pattern of 0.35 .mu.m or below, in particular of 0.30 .mu.m or below in line width, has been believed to be hardly formed by photolithography using i-ray (365 nm) as a light source without deteriorating the above properties. Photolithography using Kr-F excimer laser or Ar-F excimer layer which is shorter in wavelength than i-ray has therefore been proposed for the formation of such an ultrafine resist pattern.
Accordingly, a variety of photoresist compositions for excimer laser have been proposed.
The photolithography technique using excimer layer requires, however, expensive lithography systems and high costs in change of manufacturing lines, and the photoresist compositions for excimer laser are very fragile and need to be treated carefully.
Under these circumstances, demands have been made to provide means for sustaining the lithography technique using i-ray and to provide ultra-high definition photoresist compositions which can form an ultrafine resist pattern of 0.35 .mu.m or below, particularly of 0.30 .mu.m or below, in lithography using i-ray without deteriorating the definition, sectional shape or profile, focal depth range, and other properties.
A possible solution to the above demands is to improve alkali-soluble novolak resins to be used.
For example, Japanese Patent Laid-Open Nos. 6-202321 and 6-348007 disclose an alkali-soluble novolak resin containing units derived from m-cresol, p-cresol and 2,5-xylenol as constitutive units. The novolak resin is expected to prevent scum (resist residue after development) formation and to yield a positive photoresist composition satisfactory in definition, sectional profile, focal depth range and other properties. A photoresist composition using the novolak resin is, however, still insufficient in sensitivity, and if its sensitivity is to be increased by, for example, decreasing the molecular weight or increasing the proportion of m-cresol, the other properties, especially definition, are markedly deteriorated.
Japanese Unexamined Patent Publication No. 5-273757 discloses an alkali-soluble novolak resin obtained by condensing a mixture and formaldehyde, which mixture is composed of 30% to 98% by mole of m-cresol, 1% to 40% by mole of 3,5-dimethylphenol, and 1% to 10% by mole of polyhydroxyphenol represented by the following formula: ##STR1##
wherein R is a hydrogen or a lower alkyl group having 1 to 5 carbon atoms, and n denotes 1 to 3. The use of this novolak resin is expected to give a high-sensitivity positive photoresist composition satisfactory in heat resistance. Such a positive photoresist composition using the novolak resin invites, however, a large amount of scum, and is insufficient in definition, sectional profile, focal depth range and other properties, even though being satisfactory in sensitivity and heat resistance.
Furthermore, Japanese Patent Laid-Open No. 62-227144 discloses an alkali-soluble novolak resin obtained by condensation of phenols and formaldehyde, which phenols contain 6% to 94% by mole of m-cresol, and 94% to 6% of a compound represented by the following formula (except for m-cresol): ##STR2##
wherein X is --CH.sub.3, --C.sub.2 H.sub.5, --C(CH.sub.3).sub.3, --CO.sub.2 CH.sub.3 or --CO.sub.2 C.sub.2 H.sub.5, 3.gtoreq.n.gtoreq.1, and 3.gtoreq.m.gtoreq.1. The use of this novolak resin is expected to give a positive photoresist composition satisfactory in definition, sensitivity, and heat resistance. A positive photoresist composition using the aforementioned novolak resin is, however, inferior in sensitivity and definition, invites a large amount of scum, and is inferior in sectional profile, focal depth range and other properties, in the formation of an ultrafine resist pattern of 0.35 .mu.m or below.